Uganda says it is cutting the flow of water through its main dam at the point where Lake Victoria feeds the Nile, blaming it on widespread drought.
The Ugandan government said the flow had been cut by almost a third to allow the lake to refill as it is at its lowest level for more than 80 years.
At least 7m people are estimated by the United Nations to need immediate food aid as a direct result of the drought.
Other problems are expected to put more than 20m in need in the next year.
James Morris, head of the World Food Programme (WFP), said an area from Eritrea to Tanzania was affected by the worst drought in decades.
He said the WFP was appealing to its usual donors - Western nations and Japan - for more aid.
New drought pattern
However, he said it was also looking for increased contributions from the Gulf states and the wider Arab world.
The United Nations has warned of deaths from malnutrition
Such countries currently provide less than 1% of funding for the WFP, Mr Morris said.
For example, no funds have been received from Saudi Arabia, despite the fact that the vast majority of those in need were Muslims, he said.
New patterns of drought on the continent mean the WFP has to mount a "more sustained emergency response", he added.
He said the agency still needed $189m (£108m) to help fund aid to Kenya.
Some 3.5 million people in the country are currently thought to need help.
Uganda has dismissed claims made last month, that it has been secretly draining Lake Victoria to maintain electricity supplies.